Re: Kernel Concurrency Sanitizer (KCSAN)

[Dmitry Vyukov]

On Sat, Oct 5, 2019 at 2:58 AM Eric Dumazet <eric.dumazet@xxxxxxxxx> wrote:
> > This one is tricky. What I think we need to avoid is an onslaught of
> > patches adding READ_ONCE/WRITE_ONCE without a concrete analysis of the
> > code being modified. My worry is that Joe Developer is eager to get their
> > first patch into the kernel, so runs this tool and starts spamming
> > maintainers with these things to the point that they start ignoring KCSAN
> > reports altogether because of the time they take up.
> >
> > I suppose one thing we could do is to require each new READ_ONCE/WRITE_ONCE
> > to have a comment describing the racy access, a bit like we do for memory
> > barriers. Another possibility would be to use atomic_t more widely if
> > there is genuine concurrency involved.
> >
>
> About READ_ONCE() and WRITE_ONCE(), we will probably need
>
> ADD_ONCE(var, value)  for arches that can implement the RMW in a single instruction.
>
> WRITE_ONCE(var, var + value) does not look pretty, and increases register pressure.

FWIW modern compilers can handle this if we tell them what we are trying to do:

void foo(int *p, int x)
{
    x += __atomic_load_n(p, __ATOMIC_RELAXED);
    __atomic_store_n(p, x, __ATOMIC_RELAXED);
}

$ clang test.c -c -O2 && objdump -d test.o

0000000000000000 <foo>:
   0: 01 37                add    %esi,(%rdi)
   2: c3                    retq

We can have syntactic sugar on top of this of course.